A problem in the operation of controllable boring mechanisms exists in having the feed speed and the rotational moment applied to the boring rod so related to one another that the boring process can be carried out as quickly as possible without the boring mechanism and above all the boring rod and the boring tool being overloaded. The latter difficulty can often appear if the earth region suddenly changes, especially if one moves from a soft to a hard earth region.
The boring mechanism is usually controlled by a human operator through two levers. With one lever the operator controls the feed speed, and with the other lever the rotational speed of the boring rod and of the boring tool arranged on the end of the boring rod. For monitoring the effective feed force and the effective rotational moment, pressure gauges stand available for use by the operator, which pressure gauges indicate the momentary effective pressure in the hydraulic circuit of the hydraulic drive for the feed and for the rotation of the boring rod. If the indicated values are too low, the operator can increase the feed speed and/or the rotational moment. If the indicated values exceed critical values, the operator must immediately lower the feed speed and/or the rotational moment. If this doesn't happen or doesn't happen fast enough, because for example, the boring head encountering a layer of stone or rock, there exists the danger of breaking the rod. The expensive work tool and the rod therefore remain in the earth. The boring must be repeated, and this leads to considerable additional cost.
It is already known to provide an overload inhibiting mechanism whereby limit values for the maximum feed force and the maximum rotational moment can be pre-set. As soon as the limit values are exceeded, the feed speed and the driving rotational moment are lowered. An exceeding of the limiting values is thereby automatically avoided. Such an overload inhibiting mechanism can, therefore, prevent a damaging of the boring mechanism, of the boring rod, and of the boring work tool during a boring procedure. They do not permit, however, an optimization of the boring process, that is, to drive the boring mechanism with the highest possible speed while avoiding an overload.